Inverse feedback amplifier



y 1942- .1. M. RID-DLE, JR 2,281,618

INVERSE FEEDBACK AMPLIFIER Filed July 31; 19:59

3nventor Gttorueg of a portion of the circuit'of Patented May 5, 1942 mvnasn FEEDBACK AMPLIFIER James M. Riddle; In, West Collingswood, N. J.,

asslgnor to Radio Corporation of corporation of Delaware America, a

Application July 31, 1939, Serial No. 287,612

(cum-171i velope I, may be of anysuitable type and in the present example are shown'as triodes, the tube 3 Claims.

I The present invention relates to inverse feedback amplifiers. More particularly, it relates to impedance or resistance coupled amplifiers with inverse feedback, and has for its object to provide a high quality, low distortion feedback 'am-, plifier of that type having a minimum number of circuit components.

It is also an object of the present invention to provide a resistance coupled amplifier com prising a balanced push-pull output stage with.

driver and phase inverter stages therefor combined in one tube or having a common cathode connection and with a filter network for the inverter grid circuit, permitting the use of said common cathode connection without feedback on the phase inverter stage.

It is a further object of the present invention to provide an improved inverse feedbackamplifier including a phase inverter and a driver stage i in which the two stages may include a single tube having a common cathode connection for both stages to which the feedback may be applied for the driver stageand to the exclusion inverter stage.

of the -.It is a still further object of the present invention to provide an. improved phase inverter network for an impedancev coupled amplifier adapted for inverse feedback and improved means for controlling the frequency response of the amplifier outside of the feedback loop.

" The invention will, however, be better understood-from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing,

Figure 1 is a. schematic circuit diagram of an audio frequency amplifier embodying the invention;

Figure 2 is a similar schematic circuit diagram Fig. l showing a modification; and

Figure 3 is a graph showing curves illustrating the operation of the circuit of Fig. 2.1

Referring-to Fig. 1, the audio frequency amplifier comprises first stage amplifier elements 5 and inverter amplifier elements 6 contained in 5 having a signal input orcontrol grid H connected with an audio frequency input circuit .,I2.l3 through a volume control potentiometer It-I5 in the grid circuit of the tube.

V Biasin potential is applied between the cathode 9 and the grid ll through a common cathode connection l6, a common cathode'bias resistor IT, a ground or chassis connection I8 and the input potentiometer l3l4'-| 5. The cathode resistor I1 is maintained at a low resistance value to reduce the impedance between the cathode and ground. A desired bias potential is established therein by'bleeder current derived from a positive supply lead i9 through a connection 20 and a series bleeder resistor 2!, the negative B supply b'eing, ,the chassis or ground. With a .tubeff'l of the RCA 6Z7-G type, the,

bleeder resistor 2| may be of the order of 50,000 ohms and the resistor I! may be of the order of 500 ohms in an amplifier of the type shown.

I The output anode of the first-stage amplifier is indicated at and is coupled through an output circuit 26 and suitable impedance coupling means 21 with the grid circuit 28 and control grid 29 of one of a pair of push pull output tubes 30 and 3|. The input or control grid 32 of the tube 3| is similarly coupled to the output anode 33 of the inverter tube 6 through impedance or resistance coupling means indicated at 34. Since the impedance coupling means referred to is of scription of this type of coupling is'believed to be unnecessary.

As iswell known, the phase inverter tube 6 is employed to derive from the single channel input represented by the tube 5, a push pull or balanced output signal for the tubes- 30 and BI and for this purpose the control grid-35 ofv the in-,

a common tube envelope I with a common cathverter tube 6 is coupled to the output circuit 26 of the first stage 5 through a suitable coupling capacitor indicated at 36 and a series resistor 31 connected to the input grid lead 38. Alternatively, resistor 31- may be connected directly to line 28 thereby utilizing capacitor 96 as a couplingcapacitorfor bothigrid 29 andgrid-35,

through this connection so that both grids 35 and H are substantially equally biased.

The grid 35 receives an amplifier signal from the output circuit 26 of the first stage substantially 180 degrees out of phase with the signal applied to the grid II and in order that the signal potentials on both grids may be of the same amplitude, the grid network 31, 40, 4| is designed with proper values of resistance to reduce the amplifier signal at the lead 38 and the inverter stage grid to the desired value. In the present example the capacitor 36 may be considered'to be of the order of .0025 mid. and the resistors 31, 40 and 4| may have values of the order of 680,000 ohms, 18,000 ohms and 1.0 megohm, respectively, the ratio being such that The push-pull output stage comprising the tubes 30 and 3| may be of any suitable .type, and in the present example is of the push-pull, beam power type, the tubes 30 and 3| each having a cathode 50 connected to chassis or ground through a common self-bias resistor El and having output anodes 52 connected in balanced relation to each other through the primary 53 of an output transformer 54. The screen grids 55 of the tubes are connected to the positive supplylead i9 through a lead 56 while the output anodes receive a higher positive potential through a supply lead indicated at 51.

The secondary of the output transformer is indicated at 60 and is connected to the voice coil leads of a loud-speaker device 32 having a return circuit comprising a chassis ground connection 63 for one side of the voice coil and a chasample, the resistor 68 provides a major portion of the feedback potential drop and may be of the order of 2200 ohms, while the resistors 61 and 69 are of the order of 80 ohms each with the bypass capacitor I I- of .05 mfd., as a further aid in maintaining audio frequency disturbance voltages at a minimum. The resistors 61 and 39 are made substantially A of the value of the resistor 68 so that the disturbance effect across the resistor H by reason of undesired pickup in the switch grid networks in such a manner that the inverse sis ground connection for one side of the secondary 60.

Inverse feedback energy from the secondary 60 for correcting amplifier distortion is applied across the common .resistor IT by reason of the ground connection 64 for one side of the secondary to the ground connection l3 for the resistor H, A feedback lead 65 connected with the opposite or ungrounded side of the secondary 60, and with the cathode end 66 of the resistor I! through a pair of series connected feedback potential drop producing resistors 61 and 38 completes the feedback circuit to the input of the driver stage. The resistors may be eliminated if the lead 65 is tapped to the secondary 60 as indicated in Fig. 2 which will hereinafter be referred to.

Between the resistors 61 and 68 is connected a shunt bypass circuit to ground comprising a series controlling resistor 63 and a switch 10 shunted by a bypass capacitor II which shunts out high frequency disturbances in the switch circuit from the receiver circuits. In the open position the switch permits apredetermined amount of feedback to flow from the output circuit to the common cathode resistor l1 thereby raisingand lowering the common cathodes .3 and I0 above ground in accordance with variations in feed.-

back. When the switch is closed the feedback potential is reduced to a greater. degree thereby increasing the gain of the amplifier, and in the present example is arranged to provide substantially 3.0 DB increase in gain.

For automobile receivers, in particular, the switch I0 is operated when driving at high speeds to increase the output of the receiver to overcome the windage and other noises incident to the higher speed operation. Inthe present exfeedback voltage is filtered out of the inverter grid circuit. To this end the value of resistance forvthe resistor 4| is chosen to operate with a capacitor 12 as a filter in the coupling network 33-3|40-4|, the capacitor being connected between the common cathode connection 8-|6 and a tap I3 on the resistor 40-4! between the input grid circuit 38 of the inverter tube 3 and ground 42. In the present example, the tap connection is located between sections 40 and 4|, such that the said resistor sections and the capacitor 12 provide a grid filter substantially tying the grid 35 and the grid circuit 38 to the common cathode connection 8--| 6 for feedback potentials while at the same time the control grid grees out of phase with the signal voltage on the grid II and at substantially the same signal amplitude by reason of the furtherinclusion in the filter circuit of a relatively low resistance at 40 between the grid circuit 38 and the cathode filter capacitor section 13.

The capacitor 12 is of the order of .25 mfd.

while the resistor 40 is of the order of 18,000

the inverse feedback voltage is prevented from actuating the grid 35 and introducing distortion. The attenuation of feedback voltage with respect to grid "is in the order of 40 DB at low fre-' quencies. The attenuation may be approximately as Bythusproviding a relatively high resistance inverter grid circuit between the first stage output circuit 23 and ground, a relatively low resistance section thereof may be selected to derive a. required inverter voltage equal to the signal input voltage while at the same time the inverter input grid is substantially connected withvthe inverter cathode with respect to the inverse feedback voltage. This is provided by a minimum numexpressed ber of simple circuit components of low cost, and a simple filter and bleeder network having only two connections thereon, one for the inverter grid and one for the filter capacitor 12 to cathode.

It will be noted that the volume control device ll-l for the amplifier is provided with a tone control network comprising a capacitor 80 and a series resistor 8I- shunted across a portion of the volume control resistor l4. Thus the audio amplifier, a portion of which is included in the feedback loop, may be provided with tone control loop to properly control the tone characteristic of the amplifier without introducing distortion.

Likewise and preferably a tone control circuit may be included outside the portion of the amplifier which includes the feedback loop, for example as shown in Figure 2, between the output transformer 54 and the loudspeaker device 62 in a series connection between the secondary 60 and the voice coil 6 I. ,This comprises a suitable combination of inductance, indicated at 82, capacity,

ondary winding provided with a chassis ground connection and a feedback connection lead, a

. pair of amplifier tubes having a common cathode means which is connected outside the feedback connection to chassis including a biasing resistor connected at the cathode end with said feedback lead, means in said feedbacklead for controlling an inverse feedback voltage therethrough, means for providing a bleeder current through said cathode resistor whereby for a predetermined bias tential the resistance to chassis of said resistor may be relatively low, means providing a circuit between said amplifier tubes for eifecting phase inversion of an applied signal potential, and means including a filter in said last named connection for preventing inverse feedback potential from said common cathode connection from initially reaching said inverter stage, there.- by to prevent distortion of an applied signal through saidamplifier.

2. In an audio frequency amplifier, the combination with a balanced output amplifier stage,

indicated by the capacitor 83, and resistance, indicated by the resistor 84. The'feedback voltage is taken as in Kg. 1 through the lead 85 and the ground connection 64 on the amplifier side of the loudspeaker circuit so that the correcting network 83-83-84 is connected between the feedback connection 65 and the voice coil 8!.

Since the inverse feedback connection tends to provide the amplifier with a substantially flat frequency response characteristic, it is obvious that this may be modified without introducing distortion in the amplifier by the network referred to, and thu's'attenuate or accentuate any portion of the audio frequency signal range. For example, in the system shown, by properly adjusting the circuit constants, the high frequency signals may be divided between the voice coil 8] and the inductance 82 so that the said high frequency signals areattenuated as may be desired in the operation of said receiving and amplifying systems.

On the other hand, the network may be adjusted to accentuate or attenuate any other frequency range as desired. However, it should be understood that the direct current resistance of the inductance 82 is preferably low compared with that of the voice coil GI, and that the maximum value of resistance 84 is preferably several I means in the output circuit may be combined .with tone control means preceding the portion of the amplifier including the feedback loop, to provide any other desired output characteristic.

I claim as my invention:

1. In an audio frequency amplifier, the combination of an output transformer having a seccomprising a pair of amplifier tubes having a balanced-input circuit. and abalanced output circuit, of a combined signal amplifier and inverter tube each having an output anode resistance coupled to said balanced input circuit and'a signal input grid, means providing a com-' mon cathode connection for said amplifier and inverter stages including a common bias potential supplyresistor for said grids, means connected with said output circuit for applying a controlled inverse feedback potential across said last-named resistor, whereby the common cathode connection for said stages is varied above ground in response to variations in inverse feedback potential, and means providing an inverter voltage connection between the output circuit of a I the amplifier stage and the control grid of the inverter stage, said connection including a filter network having a resistor and a capacitor in series between said inverter control grid .and cathode connection of relatively low resistance with respect to the resistance of said inverter voltage connection, whereby for inverse feedback voltages said inverter cathode and control grid operate at substantially the same potential,

3. In a resistance coupled amplifier, comprising a driver stage, a phase inverter stage havin an input circuit, a pair of balanced amplifier tubes providing a second stage, impedance coupling means between said last named stage and said driver and phase inverter stages, means providing a common cathode connection including a low resistance bias resistor for said phase inverter and driver stages, means for applying an inverse feedback potential from said second 'stage to the cathode end of said bias resistor, and means for preventing the application of said inverse feedback potential to the inverter stage input circuit comprising a filter network providing potential drop producing coupling between said driver stage and said inverter stage for signal I potentials, and an alternating current low imfpedance connection between the inverter stage input circuit and said common cathode connection for causing said input circuit to operate at cathode potential with respect to inversefeedback potentials.

JAMES M. RIDDLE, JR. 

